1. Field of the Invention
This invention relates generally to systems and methods that provide data communication between a surface location and a downhole tool in a wellbore and more particularly to data communication from the surface to the downhole tool by utilizing mudflow variations.
2. Description of the Related Art
Wellbores or boreholes are drilled in the earth's subsurface formations for the production of hydrocarbons (oil and gas) utilizing a rig (land or offshore) and a drill string that includes a tubing (jointed pipes or a coiled tubing) and a drilling assembly (also referred to as a bottom hole assembly or “BHA”). The drilling assembly carries a drill bit that is rotated by a motor at the surface and/or by a drilling motor or mud motor carried by the drilling assembly. The drilling assembly also carries a variety of downhole sensors usually referred to as the measurement-while-drilling (“MWD”) sensors or tools. Drilling fluid or mud is pumped by mud pumps at the surface into the drill string. The drilling fluid after discharging at the drill bit bottom returns to the surface via an annulus between the drill string and the wellbore walls. The tools in the BHA perform a variety of functions including drilling the wellbore along a desired well path that may include vertical sections, straight inclined sections and curved sections. Signals are sent from the surface to the downhole tools to cause the downhole tools to operate in particular manners. Downhole tools also send data and signals to the surface relating to a variety of downhole conditions and measurements made by such tools relating to the wellbore and the formation surrounding the wellbore.
In one method, encoded signals are sent from the surface to the downhole tools using the drilling fluid column in the wellbore as the transmission medium. Such signals are usually sent in the form of sequences of pressure pulses by a pulser at the surface or by changing the drilling fluid flow rate at the surface. The changes in the flow rate are sensed or measured at a suitable downhole location by one or more downhole detectors, such as flow meters and pressure sensors, and then deciphered or decoded by a downhole controller. Such mud pulse telemetry schemes tend to be complex and can consume extensive amounts of time to transmit signals. Also, the majority of the current down linking methods where fluid flow is varied utilize rig site apparatus that require relatively precise controls of the fluid flow variations and special downhole set ups to transmit complex data.
However, many of the wells or portions thereof can be drilled by utilizing a limited number of commands or signals sent from the surface to the downhole tools, including implementing automated drilling. Consequently, a simplified telemetry method and system can be used to transmit signals to the downhole tool. Thus, there is a need for an improved method and system for transmitting signals from the surface, detecting the transmitted signals downhole and utilizing the detected signals to effect various operations of the downhole tools during drilling of wellbores.